The present invention relates to an insulator for an electrical conductor provided with an outer shield in order to achieve an insulation at the shield that insulates against DC and low frequency AC signals transforming through the conductor according to the introduction to claim 1.
When electrical conductors with an external earth, for example in the form of shielding, so-called coaxial cables intended for high frequency carrier waves are connected to audio/video equipment or to equipment for telecommunication, problems of circulating earth currents frequently arise. These currents originate from differences in AC potentials between the respective points of connection of the conductors to the equipment, which leads to disturbances in the form of humming noise and disturbing signals. Such currents occur, for example, between the exchange and transmission sections of telecommunications equipment as the likelihood of differences in earth potential is great in such electrical equipment as they are located at a large distance from one another.
To avoid such said problems, it is known to provide the conductor with an insulation that blocks DC signals and low frequency AC signals but that allows the passage of more highs frequency AC signals, which in the majority of cases means signals with frequencies that exceed 50 Hz. Such insulated conductors are commonly provided with an interruption that has a capacitive junction so that DC signals and low frequency AC signals cannot pass, whereby the point at which the capacitive connection is arranged is commonly accommodated in a metallic housing that is similarly connected to earth to prevent surrounding electromagnetic radiation from affecting the signal that is led through the conductor and, conversely, to prevent the signal being transferred in the conductor from affecting nearby equipment.
Despite a number of the insulators so far known forming excellent connections for transferring high frequency signals,.and having fully satisfactory insulation against DC and low frequency signals, there is a need to both reduce the costs of manufacturing and the size of these. One reason that contributes to why known insulators have such -bulky dimensions is that the capacitive element itself constitutes a significant portion of the total dimension of the insulator. This is mainly due to the electrical functions of the capacitive element and its general construction in which the ability to accommodate electrical charges, the so-called capacitance, is proportional to the outer surface of the electrodes and the dielectric constant of the sandwich. Insulators known to date that use standard types of capacitive element, such as condensers of the monolithic ceramic type, are mounted on a pattern card, which has the disadvantage that the insulator becomes much too bulky to be useful in practice. Compare, for example, the earthing insulator known from U.S. Pat. No. 4,987,391 (Kusiak, Jr.). To reduce the outer dimensions of the insulator, it is known to use capacitive elements that have been designed to suit the body of the insulator and its function. For example, an insulator that uses specially formed capacitive elements arranged following one another in. a row and where each one has a ring-like shape with a hole in the center arranged so that the individual electrical conductors that are arranged in the coaxial cable can pass through the insulator body is known from U.S. Pat. No. 4,559,506 (Capek). Despite this type of insulator having succeeded in reducing the outer dimensions, it is far too costly to manufacture due to the specially formed capacitive elements that have to be used with it.
One main objective of the present invention is thus to achieve an insulator of the type described in the introduction that uses a standard type of capacitive element, that is simple and cheap to produce at the same time as it only has small dimensions. A second objective of the present invention is to achieve an insulator that is shielded against electromagnetic radiation.
These objectives are achieved in the insulator according to the invention having the features stated in the claims.